1. Field of the Invention
This invention relates to a method of gravity casting in which molten metal is poured into a cavity of a mold under the force of gravity.
2. Description of the Prior Art
For example, when manufacturing a crankshaft 10 as shown in FIG. 3 by casting of ductile iron (spheroidal graphite cast iron), a gravity casting mold 1 as shown in FIG. 1 is generally used. The mold 1 is of a type to be opened and closed (the parting face is indicated at 1a in FIG. 1) and has a cavity 2 conforming to the crankshaft 10 in shape and extending in the vertical direction. A molten metal passage 3 is formed to be communicated with a lower end portion of the cavity 2 and to open upward at a pouring gate 4. Molten metal is poured into the molten metal passage through the pouring gate to fill the cavity 2 under the force of gravity.
The crankshaft 10 comprises a shaft portion 11, a pin portion 12 and a counterweight portion 13, and at the counterweight portion 13, the cross-sectional area of the molten metal path as measured in the direction perpendicular to the direction of molten metal flow increases abruptly as compared with the cross-sectional areas of the shaft portion 11 and the pin portion 12.
A large change in the cross-sectional area can generate surface defects in the cast products (crankshaft 10) due to difference in the rate of the molten metal cooling through contact with the mold 1 caused by change in the rising rate of the molten metal level in response to change of the cross-sectional area.
In order to prevent wear of the mold 1, a coating is applied to the mold surface. In the case of casting of ductile iron, wrinkle-like surface defects (indicated at A in FIG. 3) extending in the axial direction of the product are produced at portions having a relatively small cross-sectional area, e.g., the shaft portion 11 and the pin portion 12, due to the heat insulating effect of the coating. Further, similar wrinkle-like surface defects (not shown) may be generated on upper and lower flat surfaces 13a of the counterweight portions 13. Due to the heat insulating coating, the cooling rate of the molten metal is decreased to form a thin solidified layer, and the thin solidified layer is twisted by fluidized portion of the molten metal flowing at a relatively high speed, thereby generating the wrinkle-like surface defects. This phenomenon is particularly significant in casting of ductile iron, whose rate of solidification is generally low.
On the other hand, when the coating is not applied, the cooling rate of the molten metal is increased and the solidification mode of the molten metal is changed to "skin-formation type", and accordingly, though the surface defects A on the relatively small cross section portions such as the shaft portion 11 and the pin portion 12 can be reduced, wrinkle-like surface defects (indicated at B) extending in the transverse direction of the product are generated on the outer peripheral portion 13b of the counterweight portions 13 having a large cross-sectional area. Molten ductile iron has a large surface tension, and the corner between a free surface of the molten ductile iron and the mold surface has a large radius of curvature. At the outer peripheral portion 13b of the counterweight portion 13 at which the cross-sectional area corresponding to the direction of rise of the molten metal level is abruptly enlarged, the part of the molten metal in contact with the mold surface is partly solidified as it has a large radius of curvature due to lowered rising rate of the molten metal level, whereby weldline-like surface defects are generated when the molten metal level subsequently rises.